This invention relates in general to printed circuit boards, and more particularly to coaxial transmission lines on high density printed circuit boards.
Shielded transmission lines are commonly used to ensure that high speed signals and radio frequency (RF) signals are transmitted intact, without interference or corruption by other signals, and to ensure that these signals do not emanate unwanted electronic emissions. The traditional way of insulating or protecting these signals has been to use shielded cable that consists of a central conductor within a plastic dielectric material, surrounded by a braided conductor. Although this construction is extremely effective in transmitting signals over a distance, it requires cumbersome interconnects, and is not useful for transmitting signals over small distances, as on a printed circuit board, for example. Numerous methods have been employed to create a shielded signal path on printed circuit boards (PCB) over the years, with one option being to sandwich the signal conductor in the interior of the PCB between two outer ground planes. Unfortunately, this construction only protect the signal conductor on the top and bottom sides, not the two lateral sides. In addition, it requires considerable real estate on the PCB surfaces and increases the laminate thickness significantly. Incorporating shielded conductors in a high density interconnect (HDI) PCB would be a valuable addition to the art.